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CONTENTS

NAME

perlembed - how to embed perl in your C program

DESCRIPTION

PREAMBLE

Do you want to:

Use C from Perl?

Read perlxstut, perlxs, h2xs, perlguts, and perlapi.

Use a Unix program from Perl?

Read about back-quotes and about system and exec in perlfunc.

Use Perl from Perl?

Read about "do" in perlfunc and "eval" in perlfunc and "require" in perlfunc and "use" in perlfunc.

Use C from C?

Rethink your design.

Use Perl from C?

Read on...

ROADMAP

Compiling your C program

If you have trouble compiling the scripts in this documentation, you're not alone. The cardinal rule: COMPILE THE PROGRAMS IN EXACTLY THE SAME WAY THAT YOUR PERL WAS COMPILED. (Sorry for yelling.)

Also, every C program that uses Perl must link in the perl library. What's that, you ask? Perl is itself written in C; the perl library is the collection of compiled C programs that were used to create your perl executable (/usr/bin/perl or equivalent). (Corollary: you can't use Perl from your C program unless Perl has been compiled on your machine, or installed properly--that's why you shouldn't blithely copy Perl executables from machine to machine without also copying the lib directory.)

When you use Perl from C, your C program will--usually--allocate, "run", and deallocate a PerlInterpreter object, which is defined by the perl library.

If your copy of Perl is recent enough to contain this documentation (version 5.002 or later), then the perl library (and EXTERN.h and perl.h, which you'll also need) will reside in a directory that looks like this:

/usr/local/lib/perl5/your_architecture_here/CORE

or perhaps just

/usr/local/lib/perl5/CORE

or maybe something like

/usr/opt/perl5/CORE

Execute this statement for a hint about where to find CORE:

perl -MConfig -e 'print $Config{archlib}'

Here's how you'd compile the example in the next section, "Adding a Perl interpreter to your C program", on my Linux box:

% gcc -O2 -Dbool=char -DHAS_BOOL -I/usr/local/include
-I/usr/local/lib/perl5/i586-linux/5.003/CORE
-L/usr/local/lib/perl5/i586-linux/5.003/CORE
-o interp interp.c -lperl -lm

(That's all one line.) On my DEC Alpha running old 5.003_05, the incantation is a bit different:

% cc -O2 -Olimit 2900 -DSTANDARD_C -I/usr/local/include
-I/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE
-L/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE -L/usr/local/lib
-D__LANGUAGE_C__ -D_NO_PROTO -o interp interp.c -lperl -lm

How can you figure out what to add? Assuming your Perl is post-5.001, execute a perl -V command and pay special attention to the "cc" and "ccflags" information.

You'll have to choose the appropriate compiler (cc, gcc, et al.) for your machine: perl -MConfig -e 'print $Config{cc}' will tell you what to use.

You'll also have to choose the appropriate library directory (/usr/local/lib/...) for your machine. If your compiler complains that certain functions are undefined, or that it can't locate -lperl, then you need to change the path following the -L. If it complains that it can't find EXTERN.h and perl.h, you need to change the path following the -I.

You may have to add extra libraries as well. Which ones? Perhaps those printed by

perl -MConfig -e 'print $Config{libs}'

Provided your perl binary was properly configured and installed the ExtUtils::Embed module will determine all of this information for you:

% cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

If the ExtUtils::Embed module isn't part of your Perl distribution, you can retrieve it from http://www.perl.com/perl/CPAN/modules/by-module/ExtUtils/. (If this documentation came from your Perl distribution, then you're running 5.004 or better and you already have it.)

The ExtUtils::Embed kit on CPAN also contains all source code for the examples in this document, tests, additional examples and other information you may find useful.

Adding a Perl interpreter to your C program

In a sense, perl (the C program) is a good example of embedding Perl (the language), so I'll demonstrate embedding with miniperlmain.c, included in the source distribution. Here's a bastardized, nonportable version of miniperlmain.c containing the essentials of embedding:

#include <EXTERN.h>               /* from the Perl distribution     */
#include <perl.h>                 /* from the Perl distribution     */

static PerlInterpreter *my_perl;  /***    The Perl interpreter    ***/

int main(int argc, char **argv, char **env)
{
    my_perl = perl_alloc();
    perl_construct(my_perl);
    perl_parse(my_perl, NULL, argc, argv, (char **)NULL);
    perl_run(my_perl);
    perl_destruct(my_perl);
    perl_free(my_perl);
}

Notice that we don't use the env pointer. Normally handed to perl_parse as its final argument, env here is replaced by NULL, which means that the current environment will be used.

Now compile this program (I'll call it interp.c) into an executable:

% cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

After a successful compilation, you'll be able to use interp just like perl itself:

% interp
print "Pretty Good Perl \n";
print "10890 - 9801 is ", 10890 - 9801;
<CTRL-D>
Pretty Good Perl
10890 - 9801 is 1089

or

% interp -e 'printf("%x", 3735928559)'
deadbeef

You can also read and execute Perl statements from a file while in the midst of your C program, by placing the filename in argv[1] before calling perl_run.

Calling a Perl subroutine from your C program

To call individual Perl subroutines, you can use any of the call_* functions documented in perlcall. In this example we'll use call_argv.

That's shown below, in a program I'll call showtime.c.

#include <EXTERN.h>
#include <perl.h>

static PerlInterpreter *my_perl;

int main(int argc, char **argv, char **env)
{
    char *args[] = { NULL };
    my_perl = perl_alloc();
    perl_construct(my_perl);

    perl_parse(my_perl, NULL, argc, argv, NULL);

    /*** skipping perl_run() ***/

    call_argv("showtime", G_DISCARD | G_NOARGS, args);

    perl_destruct(my_perl);
    perl_free(my_perl);
}

where showtime is a Perl subroutine that takes no arguments (that's the G_NOARGS) and for which I'll ignore the return value (that's the G_DISCARD). Those flags, and others, are discussed in perlcall.

I'll define the showtime subroutine in a file called showtime.pl:

print "I shan't be printed.";

sub showtime {
    print time;
}

Simple enough. Now compile and run:

% cc -o showtime showtime.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

% showtime showtime.pl
818284590

yielding the number of seconds that elapsed between January 1, 1970 (the beginning of the Unix epoch), and the moment I began writing this sentence.

In this particular case we don't have to call perl_run, but in general it's considered good practice to ensure proper initialization of library code, including execution of all object DESTROY methods and package END {} blocks.

If you want to pass arguments to the Perl subroutine, you can add strings to the NULL-terminated args list passed to call_argv. For other data types, or to examine return values, you'll need to manipulate the Perl stack. That's demonstrated in "Fiddling with the Perl stack from your C program".

Evaluating a Perl statement from your C program

Perl provides two API functions to evaluate pieces of Perl code. These are "eval_sv" in perlapi and "eval_pv" in perlapi.

Arguably, these are the only routines you'll ever need to execute snippets of Perl code from within your C program. Your code can be as long as you wish; it can contain multiple statements; it can employ "use" in perlfunc, "require" in perlfunc, and "do" in perlfunc to include external Perl files.

eval_pv lets us evaluate individual Perl strings, and then extract variables for coercion into C types. The following program, string.c, executes three Perl strings, extracting an int from the first, a float from the second, and a char * from the third.

#include <EXTERN.h>
#include <perl.h>

static PerlInterpreter *my_perl;

main (int argc, char **argv, char **env)
{
    STRLEN n_a;
    char *embedding[] = { "", "-e", "0" };

    my_perl = perl_alloc();
    perl_construct( my_perl );

    perl_parse(my_perl, NULL, 3, embedding, NULL);
    perl_run(my_perl);

    /** Treat $a as an integer **/
    eval_pv("$a = 3; $a **= 2", TRUE);
    printf("a = %d\n", SvIV(get_sv("a", FALSE)));

    /** Treat $a as a float **/
    eval_pv("$a = 3.14; $a **= 2", TRUE);
    printf("a = %f\n", SvNV(get_sv("a", FALSE)));

    /** Treat $a as a string **/
    eval_pv("$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a);", TRUE);
    printf("a = %s\n", SvPV(get_sv("a", FALSE), n_a));

    perl_destruct(my_perl);
    perl_free(my_perl);
}

All of those strange functions with sv in their names help convert Perl scalars to C types. They're described in perlguts and perlapi.

If you compile and run string.c, you'll see the results of using SvIV() to create an int, SvNV() to create a float, and SvPV() to create a string:

a = 9
a = 9.859600
a = Just Another Perl Hacker

In the example above, we've created a global variable to temporarily store the computed value of our eval'd expression. It is also possible and in most cases a better strategy to fetch the return value from eval_pv() instead. Example:

...
STRLEN n_a;
SV *val = eval_pv("reverse 'rekcaH lreP rehtonA tsuJ'", TRUE);
printf("%s\n", SvPV(val,n_a));
...

This way, we avoid namespace pollution by not creating global variables and we've simplified our code as well.